The Alpha Centaurans were a hexapod species, known for their nervous temperaments.
The Alpha Centaurans had tentacle-like limbs, each tipped with claws. Their heads were featureless except for a large eye, with equally large optic blood vessels that nourished these eyes bulging from the skin around their heads. They were normally green in colour, though they would change in hue to match their emotion. (PROSE: The Dark Path) Red, pink and black was for anger, purple was for sadness and yellow was for fear or nervousness. Alpha Centaurans could be incapacitated by a blow to the side of the head, and struggled to right themselves if knocked off their feet. (PROSE: Legacy, The Monster of Peladon)
Despite their female-like voices, Alpha Centaurans were hermaphrodites and should properly be referred to as "it". Nevertheless, the Doctor generally used the masculine to refer to the Federation ambassador to Peladon. (TV: The Monster of Peladon) Every few years, they would develop certain urges and, if convenient, they would create an offspring. (PROSE: Legacy)
The Alpha Centaurans were known for being hypochondriacs. Alpha Centaurans never drank alcohol and could not see the appeal of it. They didn't understand humanoids, and believed many humanoid races were barbaric. Although they clearly didn't mean to do so, many of them were tactless and made remarks about various races. One Alpha Centauran commissioner in the Federation did not understand the appeal of the human game poker, believing it to be stressful. (PROSE: The Dark Path) The Alpha Centauri were known for being adept at politics. (PROSE: Mission: Impractical) The Alpha Centaurans were greatly opposed to scandals and would do anything to cover them up. (PROSE: Shakedown) Apparently they enjoyed playing table tennis, having a Table Tennis Club. (TV: Robot) A f'gan was an Alpha Centauran instrument. (PROSE: Rescue Mission) They opposed violence. (AUDIO: The Bride of Peladon)
The most common and singular article of clothing used by Alpha Centaurians was a high-collared cloak that fitted around their necks and draped over the full length of their bodies to floor level. (TV: The Curse of Peladon, Art Attack)
Though Alpha Centaurans could be found in the Earth Empire, (PROSE: Original Sin, Shakedown, PROSE: Mean Streets) on worlds such as Earth, (PROSE: Original Sin) and Proxima 2, (PROSE: Tomb of Valdemar) they became full members of the Galactic Federation. (TV: The Monster of Peladon)
During the entrance of Peladon into the Galactic Federation, Alpha Centauri acted as a diplomat. (TV: The Curse of Peladon) It later stayed on during the later war with Galaxy 5. (TV: The Monster of Peladon)
On the planet New Old Detroit, an Alpha Centauran named Hecto Shellac was such a brilliant lawyer that countless gangsters sought to use it to escape justice. Desperate for retirement, it prepared a World Bomb — capable of altering the whole planet according to the user's mind — to keep them off its back. Its dangerous actions were stopped when the Doctor activated the bomb and created a beautiful world with grass fields, supposedly because he "thought of England". Hecto survived, albeit somewhat humiliated. (COMIC: The Deep Hereafter)